The present invention relates to auxiliary devices for automatic equipment, particularly liquid level sensors.
More particularly, the invention relates to a liquid level sensor suitable as an auxiliary device in an automatic station for preparing immunologic dosages.
The determination for diagnostic purposes of substances present in the blood at very low concentrations, such as hormones, drugs, proteins etc., is possible nowadays by the use of immunologic dosage technologies (RIA, EIA, etc.,) characterised by a very high sensitivity.
The preparation of such dosages which require incubation, in a single test-tube, of several substances (e.g. unknown antigen, marked antigen, specific antibody), is carried out in several phases.
One phase is the suction, from several containers or test tubes, of liquids containing the reagents, and subsequently the dispensing of these liquids into the test tubes where the reactions are to take place.
These operations are generally performed manually by an operator using dispensing pipettes.
In recent years, within the scope of the automation process regarding all the phases of immunologic dosage preparation, computerized stations for the dispensing of dosage liquids have been developed.
A typical automatic station consists of a computer controlling the operation of:
a plotter for positioning X, Y, Z of a dispensing probe on an operation surface carrying the dosage trays; PA1 a diluter, connected to the probe, for suctioning and dispensing the liquids.
The diluter is normally made up by one or more syringes, connected by means of flexible tubes, to the probe of the plotter.
The suction and dispensing operations are carried out by moving the syringe piston up or down.
The plotter probe is called usually z-axis, and has no fixed needle, but uses tips which, at intervals, are taken from a suitable tray and automatically changed after each dispensing operation, so as to avoid any possible contamination. Usually, during operation of the preparing station, the tip collects several times from the same test tube (or container) well determined quantities of the liquid to be dispensed.
Therefore, taking into account that the liquid quantity in the test tube decreases according to the number of withdrawals, it is necessary that the system should know, before each suction, the quantity of liquid available as compared with the required quantity. That is, it is necessary to know, before withdrawing, the volume of liquid contained in the test tube. However, since the geometry of the liquid is known, it is sufficient to know one parameter only: the liquid level in the test tube. To obtain this information, it has been necessary to create a system which, during the lowering of the z-axis into the test tube, is able to:
rapidly block the movement as soon as the tip of the z-axis touches the free surface of the liquid, PA0 signal to the computer the level reached by the tip, in respect to a reference quota (z-home).
This system permits the computer to know the level, and therefore the volume, of the liquid contained in the test tube. If the quantity of liquid is sufficient to satisfy the request, the computer can at this point lower the tip with reference to the free surface of the liquid, to a depth corresponding to the volume of liquid to be withdrawn, thus avoiding lowering the tip more than necessary.
The operation could be even more refined by performing in subsequent steps, small immersions of the tip with alternate suctioning of small volumes of liquid until the total volume to be withdrawn has been reached; in this manner only a small portion of the tip is maintained immersed in the liquid. The sensors more commonly used to detect liquid levels are the capacity and resistivity types. Capacity sensors detect the difference in the dielectric constant, and therefore the capacity, when there are variations in the media in which they are immersed. The resistivity sensors make use of the conductivity of the liquid in which they are immersed, which short-circuits the two electric contacts.
Unfortunately the direct application of these sensors for the system in question would be impossible, at least for the following reasons:
(1) The different liquids to be suctioned, in order to prepare the dosages, have electric properties which differ between them; for instance, not all the liquids have good conductivity. Therefore, there is low reliability of the signals given by the sensors.
(2) The sensor should be fastened to the tip of the z-axis which, as already mentioned, is not always the same, but is automatically replaced after each dispensing operation. Therefore it would be impractical and very expensive to have all tips with the sensors incorporated.
Even one of the commonly used pressure sensors, for instance a piezometric resistivity pressure transducer, would not be directly applicable in this system. In fact, the use of such device in the dispensing system, as shown in FIGS. 1 and 2, cannot give satisfactory results due to the scarce sensitivity of the pressure sensors available on the market.
It has now been found that the above described problem can be solved by installing a new level sensor consisting of an U pipe containing water and provided with a resistivity sensor at the end of one of its branches, the other branch being connected with the terminal of the liquid suction or dispensing system of the automatic station in such a way that a minimum difference of pressure within the system results in a water level variation in both branches and consequently in the activation (or disactivation) of the resistivity sensor.